Containers have long been known which are composed of a cardboard portion and a portion of synthetic material, interconnected closely for obtaining both the advantages of cardboard and of those of the synthetic material.
The cardboard portion is formed of a grooved and cut blank which contains the different container elements, i.e. a bottom, walls and possibly a flat edge.
The plate, cut and grooved in this manner, is put in a mold, and on top of it a sheet of stretched synthetic material is placed which is thereupon thermo-formed according to a known method.
When the operation of thermo-forming is over, the finished package is kept in shape by the fact that the synthetic material adheres to the entire interior surface of the container and thus stays in place with respect to all portions of the container, just as desired.
Such a container can have all kinds of printing on it because the blank is printed upon when flat, prior to cutting and grooving. At the same time the interior faces of the cardboard are protected by a thermo-formed sheet of synthetic material against substances which would be damaging to the cardboard, such as liquids, vegetable or animal fats, etc.
Moreover, if the container has a flat edge, it is covered by a sheet of thermo-formed synthetic material. It is thus possible to close the container by means of a heat-sealed lid on the edge, in such a way that a tight container is attained which is especially adapted for packaging food products.
Experience shows that such a container has certain limitations with respect to size, especially when the bottom must be flat because twisting and warping of the entire container occur which give it a mediocre appearance and which can even have practical consequences by creating a strain on the sealing of the cover or by giving the container an undesirable instability when laid out in the show windows of stores, for example.
A well known problem for packaging products consists in providing interior separations either to maintain or arrange in a fixed position solid objects (ampules for pharmaceutical products, for example), or to insulate products which should not be mixed (liquids, foods with different tastes and the like).
A solution of this problem has been described in our U.S. patent application Ser. No. 246,706, continuation of part of our U.S. patent application Ser. No. 837,282 now abandoned. It consists in providing openings in a cardboard blank so that the relief portions of the mold for thermo-forming can traverse the blank without deforming it and can constitute the forms on which the sheet of synthetic material is applied without getting attached thereto. In practice, the relief portions thus created are formed only of synthetic material and the thickness of the sheet is chosen in such a way that it is sufficiently solid for the purpose.
To allow, on the one hand, utilizing the best qualities of cardboard and, on the other hand, the qualities of synthetic materials, it has already been considered to arrange cardboard in two parallel and spaced planes (with the edges and the bottom having small cavities), whereas the walls which are substantially perpendicular to these planes, are all of synthetic material. A container of this type is described in French Patent No. 79/12.155 of May 14, 1979.